Given a Prophet::Replica to import changes from,
traverse all the changesets we haven't seen before and integrate them into this replica.

This routine calls traverse_changesets on the 'from' replica,
passing in the most recent changeset the current replica has seen and a callback routine which calls integrate_changeset on the local replica.

That callback itself takes a callback,
after_integrate_changeset ,
which a replica implementation can use to perform some action after a changeset is integrated into a peer.
after_integrate_changeset takes a paramhash,
currently with only a single key,
'changeset'.